Generally, helium leak detectors are often used for detecting a leaking location and the amount of leakage in systems or pipings which handle vacuum or high pressures. Leak detection by the use of the helium leak detector positively utilizes the fact that air contains a negligible, substantially constant amount of helium unless helium is artificially introduced therein. More specifically, this detection method includes generally two kinds of methods, the vacuum method and the pressure method. In the vacuum method, helium is blown against a portion of a test body (system) from the outside while the system is evacuated by a pumping system in the helium leak detector alone or together with a supplementary pumping system. If a leak exists, helium flows through that portion into the system by substitution for air in the system and the helium partial pressure in the remaining gases (in the system) increases, which increases the helium partial pressure in the detector tube and the increase is detected. In the pressure method, helium or helium diluted to an appropriate concentration is sealed under pressure in a system. The gases which are leaking through a leaking location out of the system are sucked in by a helium leak detector and are introduced into the detector tube. The introduced gases which are rich in helium component by an amount of helium gas which has been substituted in place of air component.
The conventional helium leak detector comprises, as shown in FIG. 1, generally a test port 1, a throttle valve 2, a diffusion pump 3, an exhaust pipe 4, a mass spectrometer tube 5, and a rotary pump (not shown).
Gases including helium (hereinafter referred to as sample gas) drawn in from a system or a Sniffer probe (Sniffer nozzle) flow into the test port 1 and are, if necessary, subjected to reduction in pressure by the throttle valve and thereafter are sucked by the diffusion pump 3 and exhausted from the exhaust pipe 4. The mass spectrometer tube 5 is provided in the space within the diffusion pump 3 on the side of the throttle valve 2 and the helium partial pressure in this space is determined.
The throttle valve 2 is provided for maintaining the partial pressure of the air component in the sample gas (the total pressure minus the helium partial pressure) not more than 10.sup.-4 Torr in the space of the mass spectrometer tube in order to prevent the filament of the spectrometer tube from burning out. The function of the valve is to variably reduce the total pressure because of its general nature and it has no selectively throttling function, and the partial pressure ratio (component ratio) between the air and the helium therefore remains constant at the upstream and downstream sides of the valve. For example, assume that the air partial pressure P.sub.1 a applied to the test port 1 is constant and that the helium partial pressure P.sub.1 h is as follows: ##EQU1## where t equals expired time after the helium flows into the test port 1, and where .eta. equals helium concentration, then the helium partial pressure P.sub.2 h in the mass spectrometer tube 5 is ##EQU2## where P.sub.2 is the total pressure in the mass spectrometer tube 5, C.sub.v the conductance of the throttle valve 2, S.sub.v the effective pumping speed of the diffusion pump 3, V the volume of the space, including the space of the mass spectrometer tube 5, from the throttle valve 2 to the high vacuum side of the diffusion pump.
When considering the helium partial pressure P.sub.2 h except the time depending factor, it is P.sub.2 .eta. Torr and depends on the total pressure in the mass spectrometer 5 when .eta. is made constant. To raise P.sub.2 h, P.sub.2 must be made large, and a large P.sub.2 necessarily increases the air partial pressure. However, in preventing the burnout of the filament of the spectrometer tube it is not desirable that the air partial pressure exceeds 10.sup.-4 Torr. P.sub.2 h therefore must not exceed a maximum value of 10.sup.-4 .eta. Torr. As a result, there is a disadvantage that leak detection is difficult when .eta. cannot be made large, for example, when the gas load is large by the degasing of a system or the like and the leakage is small.
Furthermore, the prior helium leak detector has disadvantages that the mass spectrometer tube 5 is connected through the throttle valve 2 alone to a system and is thus liable to be contaminated with dusts, fumes, etc. in the system, with deteriorated reliability, and the throttle valve is susceptible to clogging.
Accordingly, one object of the present invention is to provide a method of detecting a leakage in a system by the use of helium, in which the system leakage detectable range of the helium leak detector is enlarged by the use of a membrane which selectively allows helium to permeate. Another object of the invention is to suppress or control air component, dusts, and fumes which are unnecessary for detection of a leakage, by the provision of the membrane, so that the exhaust system of the leak detector and also the leak detector itself are made smaller in size and simultaneously contamination of the mass spectrometer is prevented to thereby improve the reliability of the leak detector.